Transcript Microanalysis in Science and Engineering

Microanalysis in Science and
Engineering - X-ray Microanalysis
A Workshop for Middle and High School
Teachers
sponsored by
Tennessee Technological University
Center for Manufacturing Research
Departments of Chemical, Mechanical, Earth
Sciences and Curriculum and Instruction
and The National Science Foundation
Faculty
Joseph J. Biernacki (Chemical Engineering)
June 16, 2003
What will we learn
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What is X-ray microanalysis?
What interactions of electrons and matter are used?
How are the electron/matter interactions used to
generate images and compositional information?
What linkages can be made between the
“technology fundamentals” and the middle/high
school science curriculum?
What is X-ray microanalysis?
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X-ray microanalysis is the characterization of
X-ray emissions due to the bombardment of
matter with electrons.
A bit of history:
Mosely (1913) – the frequency of emitted characteristic X-ray radiation is
a function of the atomic number of the emitting element (X-ray
spectrochemical analysis)
Hillier (1947) and Marton (1941) – patented an optical microscopy/X-ray
spectrochemical analyzer
Castaing and Guinier (1949) – the first elctron microprobe
CAMECA (1956) – first commercial electron microprobe (utilized
wavelength dispersive spectroscopy technology)
Other forms of microanalysis
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When e- interact with matter, a wide range of
emissions are produced including:
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Elastic scattering emissions (backscattered electrons)
Inelastic scattering emissions
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Secondary electrons
Continuum X-rays
Characteristic X-rays
Auger electrons
UV, IR and visible light (photons)
Suggested Curriculum Links
Chemistry and Physics: quantum theory,
Bohr atom
How are characteristic X-rays
produces?
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There are many forms of interactions, however, the production
of characteristic X-rays is among the most widely used for
analytical analysis.
incident eejected e-
N
N
X-ray
N
scattered e-
Suggested Curriculum Links
Chemistry: electron orbitals,
Aufbau principle
Review of electron orbital structure
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Electrons are organized into orbitals which are filled
in increasing order of energy.
K
L
M
N
1s2
2s2p6
3s2p6d10
4s2p6d10f14
No. of e2
8
18
32
When an electron from a higher
energy orbital makes a transition to a
lower energy orbital, an X-ray is
emitted with a characteristic energy.
This energy is unique to the transition
and element.
X-ray
N
Suggested Curriculum Links
Chemistry: quantized energy
states
Transitions and nomenclature
Transitions made to a shell are given the name of the shell.
Transitions from one energy level above are designated as a,
from two levels above b, etc.
K
K excitation
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Ka
Kb
L
Ma
M
N
∞
Mb
Charcteristic X-rays - examples
Element
Mg
Ca
Fe
Ka
1.254
3.691
6.4
La
.341
.705
Suggested Curriculum Links
Physics: light diffraction,
wave theory of matter
Wavelength dispersive spectroscopy
(WDS)
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WDS utilizes the wave properties of X-rays to discern emitted X-ray energies.
X-rays, like all wave energy, can be diffracted.
Some necessary background:
Diffraction of X-rays obeys Braggs law:
nl  2d sin(q )
n=1, 2, 3, … , l=X-ray wavelength, d=the spacing between atomic planes in
a crystal, q=the diffraction angle
incident X-rays in phase
diffracted X-rays
out of phase
q
d
How does WDS work?
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In WDS the emitted X-rays are diffracted by a
crystal. The diffracted X-rays are counted by a
detector. The intensity of the diffracted X-rays is
recorded as a function of the diffraction angle.
incident eX-ray detector
diffracted X-rays
q
sample
diffraction crystal
emitted X-rays
Suggested Curriculum Links
Physics: semiconductor,
electron-hole pairs
Energy dispersive spectroscopy
(EDS)
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EDS uses a solid state X-ray detector (Li-drifted Si crystal).
Incident X-rays produce a voltage pulse that is proportional in
size to the incoming X-ray energy. In this way, information
about all energies (wavelengths) is gathered simultaneously
producing the entire X-ray spectrum without “scanning.”
incident e-
solid state
X-ray detector
sample
emitted X-rays
So what can we do with these
technologies?
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Composition point
analysis.
Composition mapping
Summary
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EDS and WDS are among the most commonly used forms of SEM
microanalysis.
Atoms ionized by incident e- radiation, emit X-rays upon relaxation.
These X-rays are unique (characteristic) to the emitting element.
Wavelength dispersive spectroscopy (WDS) utilizes diffraction to
discern between X-rays of differing wavelength (energy).
Energy dispersive spectroscopy (EDS) uses a solid state detector
to convert X-rays of different energy into voltage pulses proportional to
the incident energy.
WDS is more resolved than EDS, but data acquisition can be slower
and the instrumentation is more expensive.
Both WDS and EDS can be used to do point analysis, composition
mapping and other forms of microchemical imaging.